Well, a abilty to edit land, maybe.
But a full editor to edit a planet, seems very unlikely. Mainly down to design. Limits would be needed, for plants, water entities, maybe even mob spawn.
The plants could be randomly placed, so you can't stick them up like a fence, well if they are solid.
Buildings, yeah a issue, a auto placed turret for buildings could be a requirement so you can't create a trap that will work.
Well, its all seems to server a nice function, but designing our own planets, not likely. Though its a nice idea to create our own worlds. Hmm, maybe something similar in the future, Its one heck of a selling point, but yeah as others said, expensive. Since it would use server bandwidth, space, maybe not anything huge like a planet.
Starting to be a bit like that other virtual world. (is there not a code word we could use without actually saying the name?
secundus vita - there, latin.
)
I chose the forth though, was going to pick third, but I was afraid, I may un-balance my karma.
or 
